Influence of Ligand Environment Stoichiometry on NIR-Luminescence Efficiency of Sm3+, Pr3+ and Nd3+ Ions Coordination Compounds

Six new complexes of the ligand HQcy (-4-(cyclohexanecarbonyl)-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one) and Ln3+ ions with emission in the near-infrared (Nd3+) or visible and near-infrared (Sm3+, Pr3+) spectral regions were synthesized and characterized using various methods, including single crystal X-ray diffraction. The study demonstrated that both tris complexes [LnQcy3(H2O)(EtOH)] and tetrakis-acids [H3O][LnQcy4] can be synthesized by varying the synthetic conditions. The photochemical properties of the complexes were investigated experimentally and theoretically using various molecular spectroscopy techniques and Judd–Ofelt theory. The objective was to quantitatively and qualitatively disclose the influence of complex stoichiometry on its luminescence properties. The study showed that the addition of an extra ligand molecule (in the tetrakis species) increased molar extinction by up to 2 times, affected the shape of photoluminescence spectra, especially of the Pr3+ complex, and increased the quantum yield of the Sm3+ complex by up to 2 times. The results obtained from this study provide insights into the luminescent properties of lanthanide coordination compounds, which are crucial for the design and development of novel photonic materials with tailored photophysical properties.


Introduction
Nowadays, there is a high demand for creating new highly efficient sources of emission at the near-infrared (NIR) [1,2] or both the NIR and visible spectral regions [3,4]. One of the prominent material classes for such purposes is lanthanide coordination compounds with organic ligands. Such interest is related to optoelectronic [5][6][7][8], spectroscopic and photonic applications [9][10][11] due to the relatively high luminescence efficiency and narrow emission bands typical to trivalent lanthanide ions [9][10][11]. However, lanthanide ion's luminescence intensity is limited by Laporte selection rules [12]. Coordinating lanthanide ions with organic molecules usually significantly increases the luminescence efficiency of the ions. This happens due to the "antenna effect" [13], which is based on the electronic excitation energy transfer from a ligand to an ion, partially allowing the forbidden f → f* transition.

Crystal Structure
As all lanthanide complexes are isostructural (Table S1), structure description will only be given for [Sm(Q cy ) 3 4 ].

Tris-Complexes
The structure is a mononuclear complex (See Figure 1), where the lanthanide ion is coordinated by oxygen atoms of three ligands (O1-O6) and two oxygen atoms of solvate molecules (O7 of ethanol and O1W of water), leading to the octa-coordinated complex with the coordination polyhedron {SmO 8 } which is best described as a square antiprism. Additionally, the structure contains one solvated molecule of ethanol, which is involved in hydrogen bonding with the water molecule (O1W. . .O1S distance is 2.70 Å). Upon further analysis of the crystal packing, the presence of intermolecular hydrogen bonds between the water molecule and pyrazole fragment of the adjacent molecule of the complex (O1W. . .N2 distance is 2.83 Å) was revealed, leading to the formation of hydrogen-bonded centrosymmetric dimers (See Figure 2).

Tetrakis-Complexes
This structure is a mononuclear complex (See Figure 3), where the lanthanide ion is coordinated by oxygen atoms of four ligands, leading to an octa-coordinated negatively

Tetrakis-Complexes
This structure is a mononuclear complex (See Figure 3), where the lanthanide ion is coordinated by oxygen atoms of four ligands, leading to an octa-coordinated negatively charged complex [Sm(Q cy )4] − , while the hydronium cation (H 3 O) + is present as a counterion. The {SmO 8 } coordination polyhedron is best described as a square antiprism. Additionally, the complex lies on a two-fold rotation axis, and only half of the molecule lies in the asymmetric unit of the crystal structure. Additional proof of the electric negativity of the complex can be found from the bond length analysis of the diketone fragment (Table S2). A rather small difference in C-O and C-C bond lengths indicates that this fragment possesses delocalized negative charge. The analysis of crystal packing revealed that the hydronium cation is involved in two intermolecular hydrogen bonds with nitrogen atoms of pyrazole fragments (O1W. . .N2 distance is 2.78 Å and O1W. . .N4 distance is 2.59 Å) of two adjacent molecules of the complex, leading to the linear polymer-like crystal packing (See Figure 4). The {SmO8} coordination polyhedron is best described as a square antiprism. Additionally, the complex lies on a two-fold rotation axis, and only half of the molecule lies in the asymmetric unit of the crystal structure. Additional proof of the electric negativity of the complex can be found from the bond length analysis of the diketone fragment (Table S2). A rather small difference in C-O and C-C bond lengths indicates that this fragment possesses delocalized negative charge. The analysis of crystal packing revealed that the hydronium cation is involved in two intermolecular hydrogen bonds with nitrogen atoms of pyrazole fragments (O1W…N2 distance is 2.78 Å and O1W…N4 distance is 2.59 Å) of two adjacent molecules of the complex, leading to the linear polymer-like crystal packing (See Figure 4).     4 ] dissolved in MeCN, are shown in Figure 5. All the spectra qualitatively resemble each other. A pronounced maximum is observed at 268 nm. It was found that the coordination of lanthanide ions by this ligand leads to a significant increase in the molar extinction of the ligand environment of 40 times in comparison with the free HL ligand. Energies of the first excited singlet states were estimated as the edge of the low-energy band of the absorption spectra by a well-known tangent method [30]. For this purpose, the spectra were deconvoluted on Gaussian components. The S 1 energy values for all the compounds are similar and oscillate about 28,500 cm −1 . The increase in molar extinction is explained by an increase in the oscillator strength value of S 0 → S 1 transition due to the influence of heavy ions on ligand wave functions for the ground and excited states.    Figure 5. All the spectra qualitatively resemble each other. A pronounced maximum is observed at 268 nm. It was found that the coordination of lanthanide ions by this ligand leads to a significant increase in the molar extinction of the ligand environment of 40 times in comparison with the free HL ligand. Energies of the first excited singlet states were estimated as the edge of the low-energy band of the absorption spectra by a well-known tangent method [30]. For this purpose, the spectra were deconvoluted on Gaussian components. The S1 energy values for all the compounds are similar and oscillate about 28,500 cm −1 . The increase in molar extinction is explained by an increase in the oscillator strength value of S0 ⟶ S1 transition due to the influence of heavy ions on ligand wave functions for the ground and excited states.

Optical Absorption
Additionally, as the spectra obtained for tris and tetrakis complexes are qualitatively similar, it is clear that the optical absorption of complexes is related to ligand absorption. Notably, adding an additional organic ligand molecule leads to an increase in molar extinction by a factor of two for [Nd(Q cy ) 3

Photoluminescence
Photoluminescence (PL) spectra for all the compounds were recorded under CW excitation at 340 nm. The spectra reveal numerous narrow emission bands, originated by f  Additionally, as the spectra obtained for tris and tetrakis complexes are qualitatively similar, it is clear that the optical absorption of complexes is related to ligand absorption. Notably, adding an additional organic ligand molecule leads to an increase in molar extinction by a factor of two for [Nd(Q cy ) 3

Photoluminescence
Photoluminescence (PL) spectra for all the compounds were recorded under CW excitation at 340 nm. The spectra reveal numerous narrow emission bands, originated by f → f* transitions in ions. The interrelation of the following spectral bands with transitions was established according to the literature [44].
The PL spectra of the [Pr(Q cy ) 3 4 ] complexes are shown in Figure 7. There are narrow spectral emission bands of the Pr 3+ ion detected in the visible region of the luminescence spectra of complexes, as well as wide ligand luminescence bands in the blue-green region of the spectrum. The observed narrow emission bands centered at 485 nm, 527 nm, 594 nm, 606 nm, 645 nm, 684 nm, 706 nm and 628 nm originate

(H 2 O)(EtOH)]·(EtOH) and [H 3 O][Pr(Q cy )
transitions of the Pr 3+ ion, respectively. The presence of highly intensive ligand fluorescence in the tris complex spectrum indicates an incomplete transfer of energy to the excited states of the ion. Notably, there is a decrease in the relative intensity of the luminescence of the ligand for the tetrakis complex in comparison with the tris complex.
The spectra of both complexes of the Pr 3+ ion are similarly split by the Stark effect, suggesting the same polyhedron symmetry for both compounds. This conclusion was supported by the X-ray single crystal structure analysis (D 4d group). Unfortunately, we could not obtain the spectrum in the NIR region for the tris complex due to intensive vibration quenching on the OH groups.

(H 2 O)(EtOH)]·(EtOH) and [H 3 O][Nd(Q cy ) 4 ] complexes are shown in
There was no ligand fluorescence in the PL spectra of both complexes, which evidences the high efficiency of energy transfer from the ligand to the ion. In addition, the observed emission bands are strongly split into several sub-bands due to the Stark effect [45]. The spectra of [H3O][Sm(Q cy )4] reveal more sub-bands than the spectra of [Sm(Q cy )3(H2O)(EtOH)]·(EtOH), which may indicate that the tetrakis complex has lower coordination polyhedron symmetry in comparison with the tris complex [20]. However, as calculated by the Shape software (https://shapesoftware.com) [46], the symmetry point group of the coordination polyhedron {MO7} is a square antiprism (D4d) in both cases.   The spectra of both complexes of the Pr 3+ ion are similarly split by the Stark effect, suggesting the same polyhedron symmetry for both compounds. This conclusion was supported by the X-ray single crystal structure analysis (D4d group). Unfortunately, we could not obtain the spectrum in the NIR region for the tris complex due to intensive vibration quenching on the OH groups. The

Photoluminescence Excitation
The photoluminescence excitation spectra were obtained for all the investigated complexes with the registration wavelength of 1064 nm for Nd 3+ ion complexes, 650 nm for Sm 3+ ion complexes and 605 nm for Pr 3+ ion complexes. The excitation spectra for tris and tetrakis complexes qualitatively resemble each other pairwise (See Figure 9). The most intensive excitation for all the complexes can be achieved via the ligand excited states with the maximum located at 340 nm. However, we also observed narrow excitation bands corresponding to resonant excitation of the ions through f-f* electronic transitions. Notably, the [Nd(Q cy )3(H2O)(EtOH)]·(EtOH) complex has more intensive bands related to excitation through the ion than [H3O][Nd(Q cy )4], while for Pr 3+ ion complexes, tetrakis one has more intensive bands related to excitation through the ion.

Photoluminescence Excitation
The photoluminescence excitation spectra were obtained for all the investigated complexes with the registration wavelength of 1064 nm for Nd 3+ ion complexes, 650 nm for Sm 3+ ion complexes and 605 nm for Pr 3+ ion complexes. The excitation spectra for tris and tetrakis complexes qualitatively resemble each other pairwise (See Figure 9). The most intensive excitation for all the complexes can be achieved via the ligand excited states with the maximum located at 340 nm. However, we also observed narrow excitation bands corresponding to resonant excitation of the ions through f-f* electronic transitions. Notably, the [Nd(Q cy ) 3 4 ], while for Pr 3+ ion complexes, tetrakis one has more intensive bands related to excitation through the ion.
Molecules 2023, 28, 5892 10 of 20 Figure 9. PL excitation spectra for compounds at solid state with registration wavelengths at PL intensity maxima.

Judd-Ofelt Analysis
All the complexes were successfully investigated in terms of the Judd-Ofelt theory [47,48] and their Ωt (t =2,4,6) intensity parameters as well as the trad radiative lifetimes of the emission states of the ions are presented in Tables 1-3 and S3-S5. The general procedure for Nd 3+ and Sm 3+ complexes was the same as in [2,20,49]. The calculations included Figure 9. PL excitation spectra for compounds at solid state with registration wavelengths at PL intensity maxima.

Judd-Ofelt Analysis
All the complexes were successfully investigated in terms of the Judd-Ofelt theory [47,48] and their Ω t (t = 2, 4, 6) intensity parameters as well as the t rad radiative lifetimes of the emission states of the ions are presented in Tables 1-3 and S3-S5. The general procedure for Nd 3+ and Sm 3+ complexes was the same as in [2,20,49]. The calculations included in the transition bands are labeled in the absorption spectra of the complexes according to [44,50], see Figures S1-S3. The refractive index was set as 1.47 [51]. Notably, we carried out an analysis for praseodymium complexes via the standard JO theory with physically valid intensity parameters. The set of transitions used for the calculations was mostly chosen to minimize the root-mean-square deviation, see Tables S3-S5.  Radiative lifetimes have an increasing trend for the samarium and praseodymium tetraxis complexes. The strongest effect observed for the samarium complexes since t rad for [H 3 O][Sm(Q cy ) 4 ] is 6 times higher than that for [Sm(Q cy ) 3 (EtOH)]. Increasing of the radiative lifetime generally and mostly resulted from the stronger mixing of the 4f-states with the opposite parity states from configurations with higher energies [47]. Additionally, it is commonly considered that the parameter Ω 2 is strongly enhanced by covalent bonding and depends on the degree of symmetry of the ligand environment. Relative to the analysis of the Stark splitting and X-ray analysis for these complexes (see one for the tris complex here [20]), we hardly observed any significant changes in symmetry or coordination number, but the maxima of the NIR-luminescent bands were slightly shifted for [Sm(Q cy ) 3 (H 2 O)(EtOH)]·(EtOH) indicating the so-called "nephelauxetic effect" [52]. This peculiarity is possibly related ton the five-times-greater value of Ω 2 for the tris complex. Additionally, to the luminescent spectra of the complexes, evaluated branching ratio values for 4 G 5/2 -6 H 9/2 transition are higher than 50% for both complexes, allowing us to consider them potentially suitable for lasing.
As for the praseodymium complexes, we conclude that the radiative lifetime t rad increased by about twice for tetraxis for all discussed emission layers, viz. 3 P 0 , 3 P 1 and 1 D 2 . Since transitions from different emission layers are overlapped and strong ligandcentered luminescence occurs, we can hardly distinguish the experimental branching ratios; however, the calculated ones show a change, with branching ratios increasing for transitions of higher energies for the [H 3

O][Pr(Q cy ) 4 ] complex.
The analysis of the neodymium complexes reveals a 22% decrease in lifetime from 255 to 198 µs for tetraxis complexes, and no changes in Ω 2 parameters. Thus, the symmetry state for both complexes is the same or very close. We suppose that this behavior results from changes in the delocalization of the outer d-orbitals of ions caused by the ligand field (this is the nephelauxetic effect).

Luminescent Decays and Quantum Yields
For a better understanding of electronic excitation and relaxation processes in the investigated compounds, luminescence decays were recorded. As seen in Figure 10, the tetrakis Sm 3+ complex had a longer lifetime compared to the tris Sm 3+ complex. In particular, characteristic lifetime measured at the registration wavelength of 650 nm for [H 3 O][Sm(Q cy ) 4 ] was 55 µs, which is four times greater than the lifetime of 13 µs recorded for [Sm(Q cy ) 3 (H 2 O)(EtOH)]·(EtOH). Similarly, an increase in the luminescence lifetime was observed in the NIR region with the registration at 953 nm ( 4 G 5/2 → 6 F 5/2 ) from 14 to 66 µs for tris and tetrakis compounds. Both complexes show monoexponential behavior at visible and NIR regions, which evidences that there is only one emission center. Since transitions from different emission layers are overlapped and strong ligand-centered luminescence occurs, we can hardly distinguish the experimental branching ratios; however, the calculated ones show a change, with branching ratios increasing for transitions of higher energies for the [H3O][Pr(Q cy )4] complex. The analysis of the neodymium complexes reveals a 22% decrease in lifetime from 255 to 198 µs for tetraxis complexes, and no changes in Ω2 parameters. Thus, the symmetry state for both complexes is the same or very close. We suppose that this behavior results from changes in the delocalization of the outer d-orbitals of ions caused by the ligand field (this is the nephelauxetic effect).

Luminescent Decays and Quantum Yields
For a better understanding of electronic excitation and relaxation processes in the investigated compounds, luminescence decays were recorded. As seen in Figure 10, the tetrakis Sm 3+ complex had a longer lifetime compared to the tris Sm 3+ complex. In particular, characteristic lifetime measured at the registration wavelength of 650 nm for [H3O][Sm(Q cy )4] was 55 µs, which is four times greater than the lifetime of 13 µs recorded for [Sm(Q cy )3(H2O)(EtOH)]·(EtOH). Similarly, an increase in the luminescence lifetime was observed in the NIR region with the registration at 953 nm ( / → / ) from 14 to 66 µs for tris and tetrakis compounds. Both complexes show monoexponential behavior at visible and NIR regions, which evidences that there is only one emission center.  Figure 11. Herein, the decays have a more complicated behavior than Sm 3+ complexes and can be fitted by a multiexponential law:

Luminescent decays for [Pr(Q cy )3(H2O)(EtOH)]·(EtOH) and [H3O][Pr(Q cy )4] complexes are shown in
where τi and Ai are lifetimes and corresponding amplitudes, respectively. The measured luminescence decay is determined by  Figure 11. Herein, the decays have a more complicated behavior than Sm 3+ complexes and can be fitted by a multiexponential law:

Luminescent decays for [Pr(Q cy ) 3 (H 2 O)(EtOH)]·(EtOH) and [H 3 O][Pr(Q cy ) 4 ] complexes are shown in
where τ i and A i are lifetimes and corresponding amplitudes, respectively. The measured luminescence decay is determined by where I ir f (t ) is the instrument response function (IRF), which can be described as a double-Gaussian function with the characteristic time τ irf = 0.5 ns for the measurements in the NIR region of the spectrum and an exponential function with the characteristic time τ irf =1 ns (see Figure S1 in Supplementary Material).
where ( ) is the instrument response function (IRF), which can be described as a double-Gaussian function with the characteristic time τirf =0.5 ns for the measurements in the NIR region of the spectrum and an exponential function with the characteristic time τirf =1 ns (see Figure S1 in Supplementary Material). The Pr 3+ ion complex decays obey a three-exponential law with characteristic lifetimes of several nanoseconds ( ), hundreds of nanoseconds ( ), and microseconds ( ) (See Table 4). The longest lifetime was 1278 ns for [Pr(Q cy )3(H2O)(EtOH)]·(EtOH) and 1693 ns for [H3O][Pr(Q cy )4], and is related to ligand phosphorescence, which was observed in PL spectra (See Figure 11). We could not measure the NIR tetrakis complex decay due to the extremely low luminescence intensity. Specifically, the lifetimes obtained for [Pr(Q cy )3(H2O)(EtOH)]·(EtOH) with registration at the visible spectral region are several times higher than the lifetimes at the NIR spectral region. This phenomenon can be explained by the fact that the radiative transition → (1020 nm) is more susceptible to non-radiative relaxation on the O-H vibrations of water molecules than the transition → (606 nm) due to a small energy The Pr 3+ ion complex decays obey a three-exponential law with characteristic lifetimes of several nanoseconds ( τ 1 ), hundreds of nanoseconds ( τ 2 ), and microseconds ( τ 3 ) (See Table 4). The longest lifetime τ 3 was 1278 ns for [Pr(Q cy ) 3 4 ], and is related to ligand phosphorescence, which was observed in PL spectra (See Figure 11). We could not measure the NIR tetrakis complex decay due to the extremely low luminescence intensity. Specifically, the lifetimes obtained for [Pr(Q cy ) 3 (H 2 O)(EtOH)]·(EtOH) with registration at the visible spectral region are several times higher than the lifetimes at the NIR spectral region. This phenomenon can be explained by the fact that the radiative transition 1 D 2 → 3 F 4 (1020 nm) is more susceptible to non-radiative relaxation on the O-H vibrations of water molecules than the transition 3 P 0 → 3 H 6 (606 nm) due to a small energy difference. Shorter lifetimes were observed for tris compounds (606 nm : τ 1 = 5 ns, τ 2 = 382 ns; 1020 nm : τ 1 = 5 ns, τ 2 = 58 ns), compared to tetrakis compounds (606 nm : τ 1 = 86 ns, τ 2 = 183 ns; 1020 nm : τ 1 = 85 ns  4 ] complexes, with the registration wavelength of 1056 nm, are shown in Figure 12. The tris complex has biexponentially fitted decay with characteristic lifetimes τ 1 = 146 ns and τ 2 = 1046 ns. The tetrakis complex has monoexponential behaviour, and its characteristic lifetime is τ = 1183 ns.  Figure 12. The tris complex has biexponentially fitted decay with characteristic lifetimes = 146 ns and = 1046 ns. The tetrakis complex has monoexponential behaviour, and its characteristic lifetime is = 1183 ns. Photoluminescence quantum yield (Ф) values were measured for all the compounds under optical excitation at 365 nm. As seen from Table 4, Nd 3+ ion-based complexes have similar values of Ф = 1.3%, and Pr 3+ ion based complexes have Ф = 0.4%. However, the [Sm(Q cy )3(H2O)(EtOH)]·(EtOH) complex has Ф = 0.5% in the NIR spectral area and Ф = 1.3% in the visible spectral area, while [H3O][Sm(Q cy )4] complex has Ф = 0.4% and Ф = 2.0% in the NIR and visible regions, respectively. Therefore, adding the fourth ligand molecule to a compound leads to a significant increase in Ф for visible emissions, while there is no effect in the NIR region.

(H 2 O)(EtOH)]·(EtOH) and 1693 ns for [H 3 O][Pr(Q cy )
The 4 G5/2 → 6 H9/2 transition of the Sm 3+ ion, which is the most intensive emission band, is an electric dipole transition, while the 4 G5/2 → 6 H5/2 transition is a magnetic dipole transition. On that basis, ligand environment polarizability of Sm 3+ ion tris and tetrakis complexes was estimated as the difference between the integrated magnetic dipole and electro dipole transitions. As we observed a bigger value for the tetrakis complex (10.3 for tetrakis against 7.8 for tris), we conclude that it is more polarized, which can explain the increase in the quantum yield in the visible region for the tetrakis complex in comparison with the tris one. Photoluminescence quantum yield (Φ) values were measured for all the compounds under optical excitation at 365 nm. As seen from  4 ] complex has Φ = 0.4% and Φ = 2.0% in the NIR and visible regions, respectively. Therefore, adding the fourth ligand molecule to a compound leads to a significant increase in Φ for visible emissions, while there is no effect in the NIR region.
The 4 G 5/2 → 6 H 9/2 transition of the Sm 3+ ion, which is the most intensive emission band, is an electric dipole transition, while the 4 G 5/2 → 6 H 5/2 transition is a magnetic dipole transition. On that basis, ligand environment polarizability of Sm 3+ ion tris and tetrakis complexes was estimated as the difference between the integrated magnetic dipole and electro dipole transitions. As we observed a bigger value for the tetrakis complex (10.3 for tetrakis against 7.8 for tris), we conclude that it is more polarized, which can explain the increase in the quantum yield in the visible region for the tetrakis complex in comparison with the tris one.

Experimental Setups
Single-crystal X-ray diffraction analysis of [Ln(Q cy ) 3 (H 2 O)(EtOH)]·(EtOH) (Ln = Sm, Pr, Nd) was carried out on a Bruker D8 Venture diffractometer (MoK α radiation, ω and ϕ-scan mode), and SCXRD analysis of (H 3 O) + [Ln(Q cy ) 4 ] − (Ln = Sm, Pr, Nd) was carried out on a Bruker D8 Quest diffractometer (MoK α radiation, ω and ϕ-scan mode). The structures were solved with direct methods and refined using the least-squares method in the fullmatrix anisotropic approximation on F 2 . All hydrogen atoms were located in calculated positions and refined within a riding model. All calculations were performed using the SHELXTL [53,54] and Olex2 [55] software packages. Atomic coordinates, bond lengths, angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre with deposition numbers-CCDC 2280580-2280585, which are available free of charge at www.ccdc.cam.ac.uk (accessed on 13 July 2023).
Elemental analysis was performed on an Elementar VarioEL Cube CHNO(S) analyzer (Ele-mentar Analysensysteme GmbH, Langenselbold, Germany). The lanthanide content was determined by complexometric titration with a standard Trilon B (disodium salt of ethylenediaminetetraacetic acid) solution in the presence of Xylenol Orange as an indicator. The sample was decomposed by heating with 70% HNO 3 before titration [56].
Visible and NIR absorption spectra for all the complexes and free ligands were recorded on a JASCO V-770 (Jasco, Tokyo, Japan) spectrophotometer operating within 200-3200 nm. Concentrations of the solutions were approximately 10 −5 M. For solutions, the measurements were performed using quartz cells with a 1 cm pathlength.
Visible photoluminescence spectra and excitation spectra for the complexes and the free ligand were obtained at room temperature using a Horiba Jobin-Yvon Fluorolog QM-75-22-C spectrofluorimeter with a 75 W xenon arc lamp (PowerArc, HORIBA, Kyoto, Japan). A Hamamatsu R13456 (Hamamatsu Photonics, Hamamatsu, Japan) cooled photomultiplier tube sensitive in the UV-Vis-NIR region (200-950 nm) was used as the detector. For the NIR spectral region measurements, the same setup was used, except for the detector, which was replaced by a Hamamatsu H10330 cooled photomultiplier tube sensitive in NIR region (950-1700 nm). Luminescence decays in the visible region and NIR region of the spectrum for the complexes were obtained in solid state using the same setup equipped with a Xe flash lamp as the excitation source.
Luminescence quantum yields in the visible region were obtained using an absolute method with by a home-made setup based on a MgO-covered integrating sphere with a diameter of 180 mm and FD-10G calibrated germanium photodiode detector; a CW emitting LED (365 nm) was used as an excitation source. Each sample was measured a few times under slightly different experimental conditions, and the results were averaged. The estimated error for the quantum yields was ±10%.
For all optical measurements, the corresponding instrument response functions were taken into account. The experiments were performed in air at atmospheric pressure. Degradation of the optical properties was not observed during the experiments.

Synthesis
Commercially available reagents and solvents were used for synthesis without further purification unless otherwise stated. The HQ cy ligand was synthesized according to the previously described method [57]. Lanthanide chloride crystalline hydrates were obtained by dissolving the corresponding oxides (99.999%, LANHIT, Moscow, Russia) in extra pure hydrochloric acid.
The tetrakis complexes H 3 O[Ln(Q cy ) 4 ] were prepared according to the previously reported procedure [3,20]. For IR spectra see Figure S4. The powder XRD method was used to confirm that the powder phase composition corresponds to a single crystal (see Figure S5)

Conclusions
Tris and tetrakis coordination compounds of Sm 3+ , Nd 3+ and Pr 3+ ions were investigated pairwise in the present work. We found that adding an additional ligand molecule significantly increases the molar extinction by up to two times. In addition, it slightly changes the shapes of the Pr 3+ and Sm 3+ complexes' PL spectra in the visible region and strongly affects the shape of the Pr 3+ ion tetrakis complex PL spectrum in the NIR region. However, the polyhedron symmetry does not alter as a result of adding a fourth ligand molecule, except for Sm 3+ complexes, where we observe lower symmetry for the tetrakis complex than for the tris complex. We also report a significant difference in luminescence lifetime for tris and tetrakis complexes of Sm 3+ and Nd 3 ions. Notably, Sm 3+ tris and tetrakis complexes have similar luminescence decay behavior in the visible and NIR regions pairwise. Conversely, the short-time component disappears for the tetrakis Pr 3+ complex in the NIR region.
While Nd 3+ and Pr 3+ ions complexes have the same quantum yield values of 1.3% and 0.4% (in visible region), respectively, we observed a huge increase in quantum yield for the Sm 3+ tetrakis complex in the visible region in comparison with the tris complex (from 1.3% to 2.0%), although their quantum yield values in NIR region are practically equal (0.4-0.5%). We consider that the luminescence quantum yield increase in the Sm 3+ ion complexes is related not only to the substitution of water molecules by a ligand molecule, but also to the polyhedron symmetry change.
The findings of this study may have significant implications in optoelectronics, telecommunication, and bioimaging, where the utilization of luminescence in the NIR region of the spectrum can lead to advancements in sensing, imaging, and other cutting-edge technologies.